1. Field of the Invention
This invention relates to oil and gas drilling, and more particularly to apparatus and methods for reliably transmitting information between downhole drilling components.
2. Background
Apparatus and methods are needed to effectively transmit data along downhole-drilling strings in order to transmit data from downhole components, such as tools located at or near a drilling bottom hole assembly, to the earth's surface for analysis. Nevertheless, the design of a reliable downhole transmission system is difficult due to numerous design constraints. For example, drill strings may include hundreds of sections of drill pipe and other downhole tools connected together. Data must be transmitted reliably across each tool joint to provide a continuous path between downhole tools and the surface.
Reliably transmitting data across tool joints is difficult for several reasons. First, since the tool joints are typically screwed together, each of the tools may rotate with respect to one another. In addition, as the tool joints are threaded together and primary and secondary shoulders of the drilling tools come together, the axial alignment of tools may be inconsistent. Contacts or other types of transmission elements located at the tool joint need to provide reliable connectivity despite the relative rotation and inconsistent axial alignment of downhole tools.
Moreover, the treatment and handling of drill string components may be quite harsh. For example, as sections of drill pipe or other tools are connected together before being sent downhole, ends of the drill pipe may strike or contact other objects. Thus, comparatively delicate transmission elements located at the tool ends can be easily damaged. In addition, substances such as drilling fluids, mud, sand, dirt, rocks, lubricants, or other substances may be present at or between the tool joints. This may degrade data connections at the tools joints. Moreover, the transmission elements may be subjected to these conditions each time downhole tools are connected and disconnected. Inconsistent tolerances of downhole tools may also cause signal degradation as signals travel up and down the drill string.
Inductive transmission elements provide one solution for transmitting data between downhole tools. An inductive transmission element functions by converting electrical signals to magnetic fields for transmission across the tool joint. A corresponding inductive transmission element located on the next downhole tool converts the magnetic field back to an electrical signal where it may be transmitted along the drill string.
In selected embodiments, an inductive transmission element may include a conductor to carry an electrical current and a magnetically conductive, electrically insulating material surrounding the conductor to provide a magnetic path for the magnetic field emanated from the conductor. The magnetically conductive, electrically insulating material may reduce signal loss associated with dispersion of the magnetic field.
In certain embodiments, an inductive transmission element has an annular shape. The inductive transmission element is inserted into an annular recess formed in the secondary shoulder of the pin end or box end of a downhole tool. The annular shape allows the inductive transmission element to always be oriented correctly with respect to a corresponding inductive transmission element with which it communicates. The placement of the inductive transmission element on the secondary shoulder allows the element to be protected within the downhole tool, and reduces stress that would otherwise exist on the element if located on the primary shoulder.
The use of inductive transmission elements at tool joints may provide several advantages compared to the use of transmission elements using direct electrical contacts. For example, inductive transmission elements may provide more reliable contact than direct electrical contacts. An inductive transmission element may not require direct contact with another element, whereas the electrical contact would always require direct contact. In addition, electrical contacts may cause arcing that might ignite substances present downhole such as flammable liquids or gases.
Since a drill string may extend into the earth 20,000 feet or more, it is possible that a signal may pass through hundreds of inductive transmission elements as the signal travels up or down the drill string. The failure of a single inductive transmission element may break the transmission path between the bottom hole assembly and the surface. Thus, the inductive transmission element must be robust, provide reliable connectivity, and provide efficient signal coupling. Because signal loss may occur at each tool joint, apparatus and methods are needed to reduce signal loss as much as possible to reduce the need for frequent signal repeaters along the drill string.
Thus, what are needed are apparatus and methods to improve signal coupling in downhole inductive transmission elements.
What are further needed are apparatus and methods to reduce the dispersion of magnetic energy at the tool joints.
What are further needed are apparatus and methods to provide consistent impedance and contact between transmission elements located along the drill string.